The present invention relates to a viewing optical system for viewing an object image formed on a focusing screen, and an imaging apparatus incorporating the same.
So far, a single-lens reflex camera that is a sort of imaging apparatus using film or an electronic imaging device has comprised an image erection optical system such as a penta roof prism and an eyepiece lens system for guiding a light beam leaving that image erection optical system to the eyeball of a viewer for the purpose of viewing an object image on a focusing screen onto which a subject's image is projected by a taking lens.
Among such viewing optical systems, there is an optical system wherein the eyepiece lens system is made up of three lenses: in order from its object side, a negative lens, a double-convex positive lens and a negative meniscus lens convex on its exit pupil side, with diopter adjustment implemented by moving the double-convex positive lens in the optical axis direction, as set forth in the following patent publications 1 to 7.
In such viewing optical systems, the eyepiece lens has a symmetric refracting power profile of negative, positive and negative and the positive lens has convex surfaces on both sides so that correction of spherical aberrations, etc. are improved. And the negative lens on the exit pupil side is configured into a meniscus shape so that the principal points of the eyepiece lens are positioned near the screen side while keeping small the angle of incidence of an off-axis chief ray. An aspheric surface is applied to the double-convex positive lens on which positive refracting power concentrates for correction of various aberrations, and diopter adjustment is implemented by moving the double-convex positive lens along the optical axis.
Patent Publication 1: JP(A)2000-98266
Patent Publication 2: JP(A)2001-100115
Patent Publication 3: JP(A)2001-311881
Patent Publication 4: JP(A)2001-324684
Patent Publication 5: JP(A)2004-258234
Patent Publication 6: JP(A)2005-55874
Patent Publication 7: JP(A)2006-98948
However, the eyepiece lens system in the viewing optical systems set forth in the aforesaid patent publications 1 to 7 has a long focal length and does not lend itself to size reductions, because when it is desired to view images over a wide field of view, there is the need of increasing image size on the focusing screen.
In recent years, digital single-lens reflex cameras have caught on as imaging apparatus. However, imaging plane size is smaller than conventional Leica film's imaging plane size, and so use of the aforesaid viewing optical system is likely to make the viewing angle of field small.
An object of the invention is to provide an easy-to-view viewing optical system that is capable of viewing objects while the field of view is ensured, albeit being structurally simplified by contriving an eyepiece lens system, as well as an imaging apparatus incorporating the same.